Hydrocarbon feedstocks such as aromatic hydrocarbon feedstocks are derived from processes such as naphtha reforming and thermal cracking (pyrolysis), which can be used as feedstocks in a variety of petrochemical processes, such as para-xylene production from an aromatic hydrocarbon feedstock containing benzene, toluene and xylene (BTX), toluene disproportionation, xylene isomerization, alkylation and transalkylation. However, aromatic hydrocarbon feedstocks often contain contaminants comprising bromine-reactive compounds including unsaturated hydrocarbons, such as mono-olefins, multi-olefins and styrenes, which can cause undesirable side reactions in downstream processes. Therefore, these contaminants should be removed from the aromatic hydrocarbon feedstocks before they can be used in other processes.
Improved processes for aromatics production, such as that described in the Handbook of Petroleum Processing, McGraw-Hill, New York 1996, pp. 4.3-4.26, provide increased aromatics yield but also increase the amount of contaminants. For example, the shift from high-pressure semi-regenerative reformers to low-pressure moving bed reformers results in a substantial increase in BI in the reformate streams, which are aromatic hydrocarbon feedstocks for downstream processes. This results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from aromatic hydrocarbon feedstocks, e.g., reformate streams.
Olefins (mono-olefins and multi-olefins) in aromatic hydrocarbon feedstocks are commercially removed by hydrotreating processes. Commercial hydrotreating catalysts have proved active and stable to substantially convert multi-olefins contained therein to oligomers and to partially convert the olefins to alkylaromatics.
The clay treatment of hydrocarbons is widely practiced in the petroleum and petrochemical industries. Clay catalysts are used to remove impurities from hydrocarbon feedstocks in a wide variety of processes. One of the most common reasons for treating these hydrocarbon feedstocks with a clay catalyst system is to remove olefinic materials in order to meet various quality specifications. As used herein the term “olefinic compound” or “olefinic material” is intended to refer to both mono-olefins and multi-olefins. Olefinic compounds may be objectionable in aromatic hydrocarbons at even very low concentrations of less than a few parts per million (ppm) for some processes such as nitration of benzene. Undesirable olefins, including both multi-olefins and mono-olefins, have typically been concurrently removed from aromatic hydrocarbon feedstocks by contacting the aromatic hydrocarbon feedstocks with acid-treated clay.
More recently, molecular sieves, and particularly zeolites, have been proposed as replacements for clays in the removal of olefinic compounds from aromatic hydrocarbon feedstocks. U.S. Pat. No. 6,368,496 (Brown et al.) discloses a method for removing bromine-reactive contaminants from an aromatic hydrocarbon stream which comprises providing an aromatic hydrocarbon feedstream which has a negligible multi-olefin level and contacting the feedstream with an acid active catalyst composition under conditions sufficient to remove mono-olefinic bromine-reactive contaminants. The acid active catalyst composition comprises a crystalline molecular sieve material with a pore/channel system.
U.S. Pat. No. 6,500,996 (Brown et al.) discloses a method for the treatment of an aromatics reformate to remove olefins therefrom, the method comprising contacting the reformate with a hydrotreating catalyst to substantially convert multi-olefins contained therein to oligomers and to partially convert the olefins to alkylaromatics, separating at least some of the oligomers from the hydrotreated reformate, and then contacting the hydrotreated reformate with a molecular sieve to convert at least part of the remaining olefins to alkylaromatics. The molecular sieve is selected from the group consisting of ZSM-4, ZSM-12, mordenite, ZSM-18, ZSM-20, zeolite beta, zeolite X, zeolite Y, USY, REY, MCM-22, MCM-36, MCM-49, MCM-56, M41S and MCM-41.
U.S. Pat. No. 6,781,023 (Brown et al.), discloses a method for removing bromine-reactive contaminants from an aromatic hydrocarbon stream. The method comprises: providing an aromatic hydrocarbon feedstream that has a negligible multi-olefin level and contacting the feedstream with an unbound or self-bound acid active catalyst composition comprising self-bound MCM-22 under conditions sufficient to remove mono-olefinic bromine-reactive contaminants.
U.S. Pat. No. 6,781,023 (Brown et al.), discloses a method for the treatment of aromatics reformate to remove olefins therefrom, the method comprising contacting the reformate with a molecular sieve to convert the olefins to alkylaromatics. The molecular sieve is an intermediate pore size zeolite selected from the group consisting of ZSM-4, ZSM-12, mordenite, ZSM-18, ZSM-20, zeolite beta, Faujasite X, Faujasite Y, USY, REY and other forms of X and Y, MCM-22, MCM-36, MCM-49, MCM-56, M41S and MCM-41.
U.S. patent application Ser. No. 10/897,528 (Brown et al.), discloses a method for reducing the BI of a feed having a BI of less than about 50 and containing a linear alkylbenzene and bromine-reactive olefinic hydrocarbon contaminants. The method includes the step of contacting the feed with a catalyst comprising zeolite Y catalyst having an alpha value of about 2 to about 30 under conditions effective to reduce the amount of the bromine-reactive olefinic hydrocarbon contaminants.
Both clays and molecular sieves have limited lifetimes in hydrocarbon feedstock treatment services. The length of service correlates with the amount and the kind of olefinic compounds in the hydrocarbon feedstocks. Indeed, although clay is the less expensive of the two alternatives, it is still a significant expense and it is not uncommon for large petrochemical plants processing 1000 kilo-ton per year (KTA) reformate feed to spend more than $250,000 a year on clay.
The cost of clays and/or molecular sieves has created a need for an efficient and cost-effective method for removing contaminants from hydrocarbon feedstocks such as aromatic hydrocarbon feedstocks. The present invention solves this problem by advantageously using a combination of molecular sieve materials and clay to more efficiently remove contaminants from aromatic hydrocarbon feedstocks while extending the life of the molecular sieve materials and clay.